1. Field of the Invention
The present invention relates to an optical communication system and an optical transmitting apparatus used for this optical communication system. In particular, the present invention relates to an optical communication system that transmits an optical signal having undergone analog intensity modulation, through an optical transmission path.
2. Description of the Related Art
A technique to transmit an optical signal, which has undergone optical intensity modulation by a broadband and ultrahigh frequency electric signal, through an optical fiber is widely applied for mobile phone systems. Generally, the coverage area of a base station of a mobile phone system is 1 to 3 km in radius. As the number of mobile phone terminals increase along with the popularization of mobile phones, a compact base station with a narrow coverage area is being introduced. The compact base station is not provided with an exchange function or a traffic control function. The compact base station is a head end radio apparatus, which is established in subordination to a usual base station or a center apparatus and so forth.
The base station converts a carrier signal into an optical signal, which is then transmitted to the compact base station through an optical fiber. The compact base station demodulates the received optical signal to obtain a carrier signal and emits the carrier signal from its antenna. The compact base station converts a carrier signal received by the antenna to an optical signal, which is then transmitted to the base station through the optical fiber. A carrier signal is fundamentally a radio frequency signal transmitted/received by the antenna of a base station.
The compact base station can be downsized in since the compact base station only has to subject the optical signal to direct intensity modulation by a carrier signal. For this reason, the compact base stations are set up behind buildings or various spots in downtown areas where access frequently occurs. Compact base stations are also used to extend the service area of the base station to, such as, rural areas and colonies distant from urban areas.
Meanwhile, when the intensity of an optical signal transmitted/received between the base station and the compact base station is large (for instance, +7 dbm or more), stimulated brillouin scattering occurs in the optical fiber. The stimulated brillouin scattering generates noise and distortion in the transmission optical signal. To address this issue, Japanese Patent No. 3295029 discloses a technique to multiplex a sub-carrier signal and the main signal of a transmission optical signal.
As mentioned above, Japanese Patent No. 3295029 discloses a technique to prevent deterioration of transmission quality by multiplexing a sub-carrier signal and a main signal to be transmitted. However, this technique requires an oscillating circuit of high stability for generating the sub-carrier signal. In addition, the frequency of the sub-carrier signal must be set at a frequency in which the harmonic component of sub-carrier signal does not affect the carrier signal, which is the main signal. Moreover, the frequency of the sub-carrier signal must be set at a frequency by which intermodulation does not occur in the frequency band of carrier signal between the sub-carrier signal and the carrier signal. As explained, with the conventional art as described in Japanese Patent No. 3295029, there is a problem that the circuitry becomes larger and the control regulation becomes complicated.